The invention relates to a pressure limiting valve for fuel injection devices, particularly for fuel injection devices of internal-combustion engines constructed as common rail systems, comprising a housing with an inlet-side high-pressure connection, a return flow connection and a throttling flow connection situated between them, which has a valve seat on the inlet side and in which, in a spring-loaded manner in the direction of the closing position and displaceably as a function of the pressure in the opening direction, a valve body interacting with the valve seat as well as a piston assigned to the sealing limit arranged behind the valve seat in the direction of the return flow bound a pressure space, the piston, while sealingly moving in a sliding guide being arranged in a stable manner with respect to the coaxial valve body, and, relative to the piston and the valve body as a unit, when the pressure space is opened with respect to the high-pressure connection, the effective surfaces of the unit pressurized against the spring force being larger than when the pressure space is blocked off with respect to the high-pressure connection.
Pressure limiting valves of the above-mentioned type are known from German Patent Document DE 198 22 671 A1 in the embodiment according to FIG. 3. For a throttling flow connection, throttling elements in the form of fixed throttles are provided in the transition from the pressure space to the return flow connection, which fixed throttles are arranged in the piston and lead out to the rearward space of the piston to which the restoring spring is assigned which acts in the direction of the closing position of the valve body in the valve seat, the pressure-dependent adjustment of the piston in the opposite direction to the spring loading being limited with respect to the stop. This type of a further development in connection with common rail systems has the purpose of avoiding the buildup of an unacceptably high storage pressure in the event of a failure of the pressure control of the system and a resulting system overload. Because of the limitation of the stop, it is already difficult to reliably avoid storage pressures situated in the overload range by way of the pressure limiting valve. Furthermore, dynamic pressure courses, in particular, are critical during which the valve body, as a function of the pressure, oscillates between an upper stop limitation and a lower closing position. Relative to a given delivery and consumption quantity, thus relative to approximately stationary conditions, it seems that in this manner, by way of the fixed throttles arranged in the piston in the outflow path from the pressure space—in the case of a corresponding, but definitely critical coordination—somewhat constant flow conditions can be reached, but in the practical operation, there is still the risk of a sawtooth-type opening and closing of the pressure limiting valve with corresponding consequences with respect to its wear and to the operation of the internal-combustion engine which reacts to correspondingly fluctuating injection pressures by a correspondingly unsteady running.
In an embodiment according to FIG. 4 of German Patent Document 198 22 671 A1, it is attempted to counteract this situation such that the piston is constructed as a hollow piston and that another piston-type blocking element is arranged in the hollow space of the piston, which blocking element, in its blocking position, blocks off the throttled overflow path from the pressure space to the hollow space, and which itself is connected by way of throttle openings with the return flow connection, in which case the piston and the blocking member guided into its hollow space are jointly spring-loaded relative to their blocking position. Although by means of such a series mounting of blocking elements and throttles which are assigned to them and through which the flow can take place in the direction of the return flow, the entire pressure range to be covered between the upper limit pressure—response of the pressure limiting valve—and the lower limit pressure—closing of the pressure limiting valve—can be divided into smaller pressure ranges given by the pressure drop over the respective throttling path and thereby, relative to the steadying of the outflow conditions, can also be improved, this takes place at the expense of a relatively complicated overall construction and a very complex coordination.
Furthermore, from German Patent Document DE 198 52 409 A1, a pressure limiting valve is known which has a piston which is spring-loaded in the closing direction, moves in a bore as a sliding guide and is formed by a thin bearing needle, whose displacement path in the spring-loaded closing direction is limited with the respect to the stop, and which is guided in a radially sealing manner along an axial sealing path between a face-side stop and a connection cross-section formed by an annulus. The opening of the input-side high-pressure connection takes place as soon as the piston in the form of the slim valve needle leaves the axial sealing path as a function of the pressure, against the spring support. When the flow connection is opened up as well as when it is closed, the pressurized effective surfaces of the piston are the same, and a damping space is assigned to the piston on the spring side for the purpose of damping, which damping space is connected with the return flow and a damper piston connected with the valve needle is situated in it. Since the pressurized effective surfaces of the piston are the same because of its construction as a valve needle, which maintains the same diameter along its length, the piston oscillates corresponding to the pressure fluctuations and blocks the flow connection starting from an opening position as soon as the respective holding pressure falls under the opening pressure. There is therefore no tolerance field around the opening pressure as the reference pressure, and the flow connection is or remains closed as soon as, within the scope of pressure fluctuations, which are counteracted by the damping, the opening pressure is not reached or there is a falling below the opening pressure.
The pressure limiting valves addressed above with respect to the state of the art are therefore critical in view of high-pressure-side pressure fluctuations and make a design more difficult particularly in that the upper limit pressure dangerous with respect to damage is clearly above a working pressure which represents a still acceptable limit value for all operating ranges of the internal-combustion engine and which is to be ensured by way of the pressure limiting valve.
It is therefore an object of the invention to design a pressure limiting valve of the initially mentioned type, while its construction is simple, in such a manner that, in the case of a critical upper limit value, after the opening, a pressure reduction to a desired value can be achieved, particularly to a compromise value which is acceptable for all operating ranges of the internal-combustion engine, as long as, relative to this compromise value, an excess volume is present on the high pressure side.
According to the invention, a pressure limiting valve is provided for fuel injection devices, of internal-combustion engines constructed as common-rail systems, comprising a housing with an inlet-side high-pressure connection, a return flow connection and a throttling flow connection situated between them, which has a valve seat on the inlet side and in which, in a spring-loaded manner in the direction of the closing position and displaceably as a function of the pressure in the opening direction, a valve body interacting with the valve seat as well as a piston assigned to the sealing limit arranged behind the valve seat in the direction of the return flow bound a pressure space, the piston, while sealingly moving in a sliding guide being arranged in a stable manner with respect to the coaxial valve body, and, relative to the piston and the valve body as a unit, when the pressure space is opened with respect to the high-pressure connection, the effective surfaces of the unit pressurized against the spring force being larger than when the pressure space is blocked off with respect to the high-pressure connection, wherein the sliding guide has a connection cross-section which is overlapped by the piston and wherein the piston in a closing position of the valve body, covers the connection cross-section in the closing direction in an overlapping manner. This pressure limiting valve is distinguished by a very simple construction because the piston and the valve body form a unit which can easily be produced and because, in conjunction with the integration of the connection cross-section for the return flow into the sliding guide for the piston, as a result of this sliding guide and the piston, by means of tested devices, the corresponding sealing limit can be created also in a simple manner while the connection cross-section is variable. Functionally, the piston forms an operating piston because the admission of pressure, as a result of the pressure existing in the pressure space, causes a displacement against the spring force acting in the closing direction and the pressure in the pressure space is a function of the connection cross-section to the return flow which is opened up by way of the operating piston. Furthermore, because of the piston position which, in the closing position of the valve body, overlaps the connection cross-section in the closing direction, supplementing the different size of the pressurized effective surfaces, without any additional expenditures, the extent of the possible pressure reduction with respect to a critical, still permissible maximum pressure can be influenced just as much as the size of the pressure range, at whose leaving in the downward direction, the pressure limiting valve closes, the valve body therefore reaching its closing position with respect to the valve seat.
In order to avoid, also in the event of extreme pressure rises in the storage system, an overswinging of the unit formed by the piston and the valve body in the opening direction against the spring force, which would result in the risk of an excessively fast and extensive pressure reduction in the pressure space and thus in a bouncing of the valve body against the valve seat, it was found to be expedient to bound the opening path of the above-mentioned unit of the piston and the valve body, and therefore also the compression path of the spring acting upon this unit in the closing direction, by means of a stop.
Within the scope of the invention, it is expedient to produce the unit of the piston and the valve body in one piece, and to assign a corresponding housing bore as a receiving device to this unit, which housing bore merges into an input-side high-pressure connection in the area of the seat of the valve body.
In the opposite direction, it is expedient to design the bore in the area of the valve body in an expanded manner, particularly in an undercut manner, so that favorable flow conditions can build up in the pressure space, for which it is also expedient to further develop the transmission between the valve body and the piston as a shaft with a reduced diameter. The diameters of the mutually opposite face-side ring diameters of the piston and the valve body, which occur in the transition to the shaft, in view of a pressure relief, are of the same size but, on the whole, are selected such that, when the pressure space is opened against the high-pressure side and, corresponding to the pressure drop over the throttling path during the flow through the pressure space in the direction of the return flow, a pressure-dependent adjusting force occurs which acts against the spring force and also compensates the spring hardening occurring with an increasing adjustment of the piston against the spring force.
The surface situated in the transition from the shaft to the piston forms the face-side area of the piston which, when the pressure limiting valve is closed, overlaps the sliding guide and, together with the sliding guide, forms the sealing limit in the transition between the pressure space and the connection cross-section overdriven by the piston to the return flow.
Relative to the dimensions occurring in practice, it should be taken into account that also very small diameters to approximately 2 mm can be used for the valve body, while the diameters for the piston are of a dimension of approximately 4 mm. These result in high manufacturing-related requirements, particularly also in view of the fact that, for coordinating the pressurized effective surfaces, further gradations are necessary.
As a further development of the invention, it was therefore found to be expedient to assign to the unit consisting of the valve body and the piston, on the spring side, a damping space connected with the pressure space, and with respect to its fraction of the pressurized effective surfaces, adapt this damping space by way of the diameter of an axial compensating pin to the requirements. This results in the possibility of changing, particularly for the piston, but also for the valve body, to larger diameters, and of achieving, in the case of a small cross-section of the compensating pin, high closing pressures, so that an excessively long opening of the valve at a maximal stroke, or the freeing of excessively large connection cross-sections is avoided. In the case of such a further development, the spring can constructionally be supported against the pin.
Further details and characteristics of the invention are contained in the claims. In addition, the invention will be explained in the following by means of a pressure limiting valve according to the invention which is illustrated in a schematic sectional view.